This invention relates to a method and an apparatus for use in processing a substrate to remove or form a film on a processing region of the substrate. It should be noted throughout the instant specification that the method and the apparatus are used in manufacturing a photomask, a photomask blank, a reticle, a reticle blank, a reticle testing substrate, a semiconductor substrate, a magnetic disk, a color filter, and the like, although description will be mainly made about the photomask blank and the reticle testing substrate.
Recent requirements have been directed to a photomask blank which can delineate a fine pattern at a high precision and a high resolution by the use of a photolithography technique. To this end, a phase shift method has been proposed in Japanese Unexamined Patent Publication Sho 58-173744. With this method, it is possible to manufacture a phase shift photomask blank which comprises a transparent substrate, a plurality of opaque patterns on the substrate, and a transparent film on the substrate and the patterns. The transparent film is selectively etched into transparent patterns by a photolithography technique to manufacture the phase shift photomask.
Herein, the transparent film is formed by a spin coating technique in consideration of flatness of the transparent film. A flat transparent film is produced by the use of the spin coating technique at a center portion of the transparent film and may be, for example, a resist film. However, the flatness of the transparent film is not kept at a peripheral portion of the film because the transparent film tends to become thick at the peripheral portion due to the spin coating technique, as known in the art. In other words, the peripheral portion of the transparent film has an irregular thickness when the spin coating technique is used to form the transparent film.
Moreover, the periphery of the phase shift photomask is intended to be attached or supported by other equipment, such as a support member when optically coupled to an optical system for the photolithography. This means that the periphery of the phase shift photomask is preferably flat as, otherwise, optical adjustment of the phase shift photomask becomes difficult because of the irregular thickness of the peripheral portion of the transparent film.
In order to avoid the irregular thickness of the peripheral portion, disclosure is made in Japanese Patent Publication No. Sho 58-19350, for removing the transparent film at the peripheral portion thereof from the substrate. More specifically, a solvent for the transparent film is discharged from a nozzle onto the peripheral portion of the transparent film. However, the solvent is not confined only to the peripheral portion of the transparent film. In this case, the peripheral portion of the transparent film is referred to as a processing region while the remaining portion is an unprocessing region. With this method, the solvent undesirably spreads from the processing region to the unprocessing region. As a result, the unprocessing region of the transparent film is often dissolved by the solvent in addition to the processing region.
Alternatively, a reticle testing substrate is used for optically testing a reticle which has a pattern on a reticle surface. In other words, the reticle testing substrate serves to determine whether or not a failure is included in the pattern on the reticle. To this end, such a reticle testing substrate comprises a quartz substrate having a substrate surface, an opaque film of chromium deposited on a peripheral region of the substrate surface, and a resist film uniformly coated on the substrate surface and the opaque film. In this situation, the pattern on the reticle surface is optically transcribed onto the resist film coated on the substrate surface of the reticle testing substrate by the use of a step and repeat technique. Such optical transcription of a pattern is carried out by adjusting a focus of an optical system to the opaque film. Thus, a resist pattern is formed on the substrate surface and is identical with the pattern on the reticle to be tested.
Under the circumstances, light is emitted onto the resist pattern through the reticle testing substrate so as to form a pattern image of the resist pattern by the light and to check whether or not a failure is included in the pattern image. When a failure is found on the pattern image, the reticle which corresponds to the reticle testing substrate is rejected as a faulty reticle and, otherwise, the reticle is accepted as a non-faulty reticle.
Herein, it is to be noted that the opaque film of chromium is left only along the periphery of the quartz substrate and may not be always precise in size. In order to leave the opaque film only along the periphery of the quartz substrate, a chromium film is deposited on the quartz substrate and the photoresist layer is left only on a peripheral portion of the chromium film. Thereafter, the chromium film is etched by an etchant to leave the opaque film along the periphery of the quartz substrate. In this way, it is very simple to leave the photoresist film only along the peripheral portion, namely, the processing region of the chromium film.